Peroxide cured (meth)acrylate resin coating compositions find a wide variety of uses in industry and are commonly used as coatings and sealants. The coating compositions are conventionally provided as two-component compositions which are mixed just prior to use. In typical formulations, Part A comprises the (meth)acrylate monomers and Part B comprises a peroxide initiator. Metal complex driers, such as cobalt naphthenate or cobalt octoate, are often added to the compositions as cure accelerators in these systems. Cobalt driers undergo oxidative reactions that reduce the decomposition temperature of the peroxide but by themselves are not enough to effectively cure the coating at the surface.
Peroxide cured coating compositions containing (meth)acrylate functional monomers and oligomers are known to the industry to not cure effectively in air due to oxygen inhibition.
Numerous attempts to develop additives to allow for effective curing in air have resulted in materials that were unstable in air, unstable when mixed with metal drier, had poor surface properties, or had viscosities impractical for use in a solvent-free coating system.
Paraffinic and other waxes are often added to these coatings compositions to provide an oxygen barrier formed through migration and crystallization of the waxes at the coating surface. For example, JP 2007197598 and JP 08231655 disclose peroxide cure coating systems comprising paraffin wax. The presence of the wax, however, detracts from the surface properties of the cured composition and decreases intercoat adhesion.
Crosslinking monomers such as dicyclopentenyl and dicyclopentenyloxy alkyl ester derivatives are known to the industry to be good reactive diluents and binder resins that also effectively promote surface cure in peroxide cured (meth)acrylate-based coatings in the presence of metal driers due to their sensitivity for oxidative reactions. However, these monomers are also known in the industry to be volatile and odorous which may present safety and handling concerns.
Other crosslinkable resins such as allyl ether derivatives including polyallyl glycidyl ether (PAGE) derivatives, allyl ester derivatives and allyl urethane derivatives are also known to effectively promote surface cure in peroxide cure (meth)acrylate-based coatings. Lower molecular weight resins are good reactive diluents but higher molecular weight resins like PAGE derivatives and polyallyl urethanes are too high in viscosity for some coating applications such as concrete sealers. These materials are also oxygen sensitive and undergo oxidative reactions in the presence of metal driers to generate hydroperoxides at the surface of the coating. Therefore, these allyl ether and PAGE derivatives present package stability issues if they are packaged with the metal drier in the resin component of a two-component peroxide cure (meth)acrylate-based coating composition. To avoid the stability issues, coating formulators typically have to package the surface cure promoting material as a third component or package the metal drier as a third component which is undesirable. Volatile non-aerobic sensitive materials such as oximes can be added to block metal drier oxidative reactions in the container and extend package stability. Subsequently, when the coating is applied to a substrate the oximes volatilize but they also slow down the rate of cure at the coating surface, which may lead to coating defects and increased tack-free times.
U.S. Pat. Nos. 4,520,184 and 5,567,788 disclose coating compositions containing allylic functional ethers and esters including PAGE derivatives comprising 20 allyl groups per chain, which contain easily abstracted allylic hydrogens that absorb oxygen and generate hydroperoxide radicals at the surface of the coatings. These materials are used as both reactive diluents and surface cure additives in two-component peroxide cure (meth)acrylate-based coatings. The allyl functional ethers and esters cause formulation issues due to their instability in the presence of free-radical cure accelerators such as metal driers in the resin component, which requires the formulator to either develop a complex stabilizer package or to separate the accelerator. When added to the peroxide initiator component of a two-component system, the addition of the functional ethers and esters result in a limited shelf life.
Oxygen inhibition observed in two-component (meth)acrylate-based peroxide free radical cured coatings containing allyl functional ethers and esters exhibit a difference in cure kinetics between allylic unsaturation and vinyl unsaturation in the base (meth)acrylate coating resins. The slower rate of cure of the allyl unsaturation at the surface of the coating can result in coating defects such as wrinkling, cratering, and orange peel. Additionally, although the allyl functional additives provide tack-free cure, the cured surface does not exhibit good scratch/mar resistance or solvent resistance.
Other methods for overcoming oxygen inhibition have been attempted. For example, U.S. Pat. No. 5,164,127 discloses curing the coating in an inert atmosphere or eliminating oxygen by injecting the coating into a closed mold and curing the in the mold.
U.S. Pat. No. 6,395,822 discloses the use of azonitrile-based free radical initiators in place of peroxide initiators, which are not sensitive to oxygen inhibition. The azonitrile initiators cannot be decomposed at ambient temperatures by accelerators and must be thermally cured.
U.S. Pat. No. 6,835,759 discloses the use of a dual UV photoinitiator/thermal peroxide cure system to eliminate cure rate differences between through cure and surface cure.
Other methods have been disclosed in U.S. Pat. Nos. 4,263,372; 5,387,661; 5,721,326; 6,559,260, “Synthesis and Properties of Acrylate Functionalized Alkyds”, N. Thanamongkollit, M. Soucek, University of Akron Polymer Engineering Department, Progress in Organic Coatings, Vol. 73, Issue 4, April 2012, pp. 382-391; “Tung-based Reactive Diluents for Alkyd Systems: Film Properties,” K. Wutticharoenwong, J, Dzickowski, M. Soucek, University of Akron Polymer Engineering Department, Progress in Organic Coatings, Vol. 73, issue, 4, April 2012, pp. 283-290, and “Synthesis of Twig Oil-based Reactive Diluents,” K. Wutticharoenwong, M. Soucek, University of Akron Polymer Engineering Department, Progress in Organic Coatings, Vol. 295, 2010, pp. 1097-4106.
There is a need in the industry for additives that may avoid the problems caused by oxygen inhibition without some or all of the shortcomings identified above. The development of a low viscosity, low odor crosslinker that enables two-component, solvent-free, peroxide cured (meth)acrylate resin coating compositions to cure effectively in the presence of oxygen at both ambient and elevated temperatures and exhibit good surface properties is desirable.